Let's say you are using the vars plugin to create variables. Notice in this example that var2 contains null and var3 contains no value.

---
- hosts: localhost
  vars:
    var1: "Hello World"
    var2: null
    var3:
...

Or like this, using set_fact.

---
- hosts: localhost
  tasks:
  - set_fact:
    var1: "Hello World"

  - set_fact:
    var2: null

  - set_fact:
    var3:
...

The debug module can be used to print the value of each variable.

---
- hosts: localhost
  tasks:
  - set_fact:
    var1: "Hello World"

  - set_fact:
    var2: null

  - set_fact:
    var3:

  - debug:
      var: var1

  - debug:
      var: var2

  - debug:
      var: var3
...

Something like this should be returned. Notice that both var2 and var3 contain no value.

ok: [localhost] => {
    "var1": "Hello World"
}

ok: [localhost] => {
    "var2": ""
}

ok: [localhost] => {
    "var3": ""
}